The present invention relates to silylphosphates and more particularly the present invention relates to an improved process for producing silylphosphates where in the reaction ingredients there is present a catalytic amount of a silylphosphate to initiate the reaction.
The compositions of diorganopolysiloxane polymers is well known. Such polymers in the case of heat vulcanizable silicone rubber compositions comprise taking a high molecular weight diorganopolysiloxane gum, incorporating filler into it and other additives, and then curing the gum in the presence of a peroxide catalyst at elevated temperatures.
In the case of room temperature vulcanizable silicone rubber compositions, the process for forming them comprises taking a silanol-terminated diorganopolysiloxane polymer, a filler, a cross-linking agent such as, an acetoxy or alkoxy functional silane, and a catalyst such as a metal salt of a carboxylic acid or a titanium chelate in the case of alkoxy functional silanes, wherein the ingredients are mixed in anhydrous conditions and when exposed to atmospheric moisture cures to a silicone elastomer. Such a room temperature vulcanizable silicone rubber composition is known as a one-component system or a one-package system since all of the ingredients are incorporated into the composition and all that is needed to cure the composition is to expose it to atmospheric moisture.
There is also known two-component room temperature vulcanizable silicone rubber compositions. Such compositions comprise a silanol-terminated diorganopolysiloxane polymer and a filler wherein the filler and silonal material are packaged separately in a first component. Then in the second component, that is packaged separately, there is present an alkyl silicate or alkyl orthosilicate and a metal salt of carboxylic acid. When it is desired to cure the composition the two components are mixed and applied and at room temperature to cure to a silicone elastomer. Such a composition which is a two-component room temperature vulcanizable silicone rubber composition will cure either in the presence or absence of atmospheric moisture. The main ingredient in either the heat curable composition or the room temperature vulcanizable silicone composition is the diorganopolysiloxane polymer which polymer is a viscous mass at room temperature or at elevated temperatures, but which cross-links during cure of the above composition to form a silicone elastomer.
The process for forming such diorganopolysiloxane polymers is also well known. Such a process generally comprises taking diorganodichlorosilanes and hydrolyzing them where the organo groups can be any monovalent hydrocarbon radical or halogenated monovalent hydrocarbon radical to form a mixture of low molecular weight diorganopolysiloxane polymers and cyclicsiloxanes.
Accordingly, to proceed further in the process, there is added a cracking catalyst to such a hydrolyzate such as, potassium hydroxide and the resulting mixture is heated at elevated temperatures, that is, temperatures above 100.degree. C., to preferentially convert most of the low molecular weight linear polysiloxanes and cyclicsiloxanes to cyclotetrasiloxanes. It can be appreciated in the initial hydrolyzate and even during the cracking process that there are other cyclicpolysiloxanes such as, cyclotrisiloxanes, cyclopentasiloxanes, cyclohexylsiloxanes, and etc. However, it has been found for nonfluorinated polymers that the most desirable cyclopolysiloxane for the formation of linear diorganopolysiloxane polymers is a cyclotetrasiloxane. Accordingly, after there has been obtained sufficient conversion of the hydrolyzate to the cyclotetrasiloxanes such cyclotetrasiloxanes are collected in essentially pure form by distillation procedures.
In accordance with the desired type of substitution that is desired in the final linear diorganopolysiloxane polymer, such cyclotetrasiloxanes are taken and then there is added to them from 5 to 500 parts per million of a strong alkali metal hydroxide as a catalyst. The preferred catalyst is potassium hydroxide, however, sodium hydroxide can be used in certain instances. In certain cases such as with fluoro-substituted cyclotetrasiloxanes there can also be used a stronger alkali hydroxide catalyst such as, for instance, cesium hydroxide. However, for the preparation of other than fluorosubstituted linear diorganopolysiloxane polymers, the cyclotetrasiloxanes are taken, there is added into them the appropriate amount of alkali metal hydroxide, and there is also added to them the appropriate amount of chain-stoppers such chain-stoppers being low molecular weight linear triorganopolysiloxane polymers. An example of a suitable chain-stopper is, for instance, hexamethyldisiloxane, octamethyltrisiloxane and etc.
Accordingly, in the case for the preparation of linear diorganopolysiloxane polymers the above mixture is then heated at temperatures above 100.degree. C., and preferably at temperatures above 150.degree. C. to form a linear diorganopolysiloxane polymer. It should be noted in such reaction, which is known as an equilibration reaction, that the rings of the cyclopolysiloxanes are broken and the cyclopolysiloxanes react with each other to form a linear diorganopolysiloxane polymer. However, when about 85% of the cyclotetrasiloxanes have been converted to a linear diorganopolysiloxane polymer, it has been found that no more of the linear diorganopolysiloxane polymer is formed. Accordingly, at about the 85% conversion level the reaction is usually terminated by cooling the reaction mixture, neutralizing the alkali metal hydroxide catalyst with an acid, and then venting off or stripping off the unreacted cyclics, which cyclics can be recycled in another equilibration reaction. It should be noted that the molecular weight and viscosity of the final linear diorganopolysiloxane polymer that is formed as a result of the above reaction will depend on the amount of chain-stoppers that is utilized in the reaction mixture since this will basically control the average molecular weight of the polymers that are formed.
To produce a silanol-terminated diorganopolysiloxane polymer basically the same reaction is used except for a chain-stopper there is utilized low molecular weight silanol-terminated diorganopolysiloxane polymers or water.
One important part of this process for forming such linear diorganopolysiloxane polymers, whether silanol-terminated or triorganosilyl-terminated, is the neutralization of the basic catalyst that is utilized in the equilibration reaction. It is desired that the catalyst be neutralized since if it is not neutralized, even at room temperature, it will cause reversion of the linear diorganopolysiloxane polymer to the corresponding cyclotetrasiloxanes over a period of time. In addition, if the alkali metal hydroxide catalyst is not neutralized the cured elastomer that is formed from such a polymer will not have as good physical properties as would be desired and would be susceptible to degradation due to reversion again in the presence of heat or high humidity.
Many known types of acids are known to have been utilized for such neutralization procedures. However, one difficulty with strong acids such as hydrochloric and sulfuric acid is that care has to be taken that the amount of acid that is added is exactly the appropriate amount to carry out the neutralization. It has been found that excess acid in such linear diorganopolysiloxane polymers will cause degradation of the polymer similar to the type that is obtained when there is present excess base or excess alkali metal hydroxide in the linear diorganopolysiloxane polymer. Accordingly, the exact neutralization of the alkali metal hydroxide with a strong acid is very difficult and time consuming to carry out in a large plant batch reaction mixture.
The same kind of difficulty is experienced with mild acids such as, acetic acid. In addition, recently there evolved a semi-continuous or continuous process for the production of linear diorganopolysiloxane polymers. Accordingly, in such processes it becomes very important to have a neutralizing agent which will quickly and continuously neutralize the alkali metal hydroxide in the equilibration reaction mixture without necessitating the weighing out of exact amounts of neutralizing agent for the neutralization reaction. Accordingly, it was highly desirable to utilize a buffering type of acid for neutralizing the alkali metal hydroxide in the foregoing equilibration reactions. An example of a good buffering acid for the neutralization reaction of alkali metal hydroxides is phosphoric acid. Such an acid has the appropriate buffering action with alkali metal hydroxides and as such exact quantities of it do not have to be metered out in the reaction mixture to completely neutralize the alkali metal hydroxide without making the polymer strongly acidic. Another triprotic acid having this property such as, for instance, is arsenic acid but arsenic acid is undesirably toxic and has oxidizing and reduction properties that make it undesirable.
Phosphoric acid is cheap and non-toxic and accordingly is highly desirable as a neutralizing agents in the foregoing equilibration reactions. However, phosphoric acid has one disadvantage, that is, it is not soluble in the linear diorganopolysiloxane polymer or in the cyclotetrapolysiloxanes. Accordingly, because of its limited solubility it takes time for it even with good agitation to mix with the alkali metal hydroxide so as to neutralize it in an equilibration reaction mixture. This disadvantage which is noticeable for batch type of equilibration reactions causes even more of a problem in its use in the neutralization of continuous polymerization reactions. Accordingly, because of its limited solubility phosphoric acid is utilized with difficulty in neutralizing alkali metal hydroxide catalysts in continuous equilibration reactions for the formation of linear diorganopolysiloxane polymers.
Accordingly, it was highly desirable to have a soluble form of phosphoric acid so that it could be utilized as a neutralizing agent in equilibration reactions for forming linear diorganopolysiloxane polymers whether such processes were continuous or batch-wise. Such a soluble form of silyl phosphates is disclosed in the patent application of Razzano, Ashby, Peterson, Docket 60SI-7, entitled "Silyl Phosphates as Neutralizing Agents for Alkali Metal Hydroxides", Ser. No. 854,562, filed on Nov. 25, 1977. While such silyl phosphates are advantageous for both the batch and continuous neutralization of alkali metal hydroxides they are formed only with difficulty from phosphoric acid. The basic process for producing such silyl phosphates is to react low molecular weight diorganopolysiloxane polymers with phosphoric acid at elevated temperatures so as to form the desired silyl phosphate product. Another process, and more desirable, which produced a soluble form of silyl phosphate was the reaction of a linear diorganopolysiloxane polymer of low molecular weight with a cyclotetrasiloxane and phosphoric acid at elevated temperatures. However, in both such reactions of polysiloxanes with phosphoric acid it was found that the reaction (even at elevated temperatures) would not initiate for anywhere from 30 minutes to 2 hours and then would suddenly initiate with a very rapid and uncontrollable evolution of water which was vaporized at the reaction temperature with the rate of formation of water vapor causing the reaction mixture to boil out of the reaction vessel. Accordingly, it was highly desirable to have a reaction of the polysiloxanes in phosphoric acid in which the reaction initiated quickly and proceeded smoothly at a rapid rate and such that the reaction was not violent.
Accordingly, it is one object of the present invention to provide for an efficient process for producing silyl phosphates from phosphoric acid.
It is another object of the present invention to provide for a catalyst for the reaction of phosphoric acid with polysiloxanes such that the reaction will initiate promptly and proceed smoothly and under control.
It is an additional object of the present invention to provide for an efficient process for the production of silyl phosphates by the reaction of phosphoric acid with polysiloxanes such that the reaction initiates quickly and proceeds smoothly in a short period of time.
These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.